Screw type implants are implants having threaded outer surfaces and used as anchoring members for different prostheses, such as dental and orthopedic prostheses. This type of implant is screwed into a borehole arranged in the bone tissue of a bone tissue structure at a site where a prosthesis is required. The borehole may be formed into a shape generally corresponding to the shape of the implant, although slightly smaller in size. These implants may be provided with self-cutting edges to cut one or more internal threads in the inner wall of the borehole during the screwing in of the implant. If there are no self-cutting edges, the bore must be internally threaded before insertion of the implant.
Bone tissue has two components, cancellous bone tissue and cortical bone tissue. The major part of a bone usually is built up with the cancellous bone tissue, which is a relatively soft tissue in the interior of the bone. The cortical bone tissue is harder and normally forms a relatively thin layer surrounding the cancellous bone. Thus, in their final position, screw implants of the type described would typically be in contact with cancellous bone tissue along a larger part of its length, and with cortical bone tissue only at a shorter portion at one end of the implant.
When a screw type implant is in anchored position in the bone tissue, a superstructure for carrying a prosthetic part may be secured to the implant. In the case when a screw implant is used to secure a dental prosthesis, the superstructure will typically comprise an abutment or transmucosal component, which engages the implant to bridge the gingiva overlaying the maxilla or mandible at the implant site. The prosthetic part, e.g., a crown, a bridge, or a denture, is then secured to the abutment. The implant could also be formed integrally with a superstructure, such as a transgingival component, on which for example a crown is directly secured. A problem occurring when using many screw type implants is referred to as the bone resorption problem. Bone resorption is a term used for a process in which once an implant is installed in the bone tissue, the bone surrounding the implant tends to degenerate. This is highly undesired, since a diminished amount of bone surrounding the implant will lead to diminished stability and sometimes result in failure of the prosthesis. This is particularly the case because bone resorption primarily occurs in the cortical bone, which, as mentioned above, is the hardest part of the bone. Once bone resorption exists, secondary problems may also appear. Such secondary problems, particularly related to dental implants, are for example deposition of plaque, resulting in inflammation in the gingival tissue surrounding the implant, or down-growth of gingival tissue along the exposed end of the implant. Also, the aesthetic appeal of the implant is undermined by bone tissue resorption, which is an important drawback when the implant is intended for dental applications since dental prostheses form part of the field of cosmetic surgery.